Membrane electrocells (ECs) are used primarily by the chlor-alkali industry and for sea water desalination. Commercially available devices are made in a flat configuration with stack designs similar to that of filter presses. An EC, in its simplest form, is made up of an anode, one or more membranes, and a cathode. The membrane sheets are sealed so that fluid chambers are formed in layers between the electrodes. ECs are capable of separating chemicals because the membranes contain microscopic pores which allow chemical species to diffuse from one chamber to another. Such diffusion is slow. However charged species (ions) can be pulled through the pores more quickly if an electrical field is applied between the electrodes. For example, in a two-chambered cell (one membrane), if a sodium sulfate solution is fed into an anode chamber, an electric field will cause sodium ions to move through the membrane into a cathode chamber. This migration will produce a sulfuric acid solution in the anode chamber and a sodium hydroxide solution in the cathode chamber. The efficiency of this scheme is enhanced with membranes which specifically allow ions of a specified charge to pass. Therefore, in this example, a membrane would be used which selectively passes positive ions. This would inhibit movement of hydroxide ions from the cathode chamber into the anode chamber, thus saving electrical energy.
Commercial ECs are generally either electrodialysis cells or salt-splitting cells. Electrodialysis cells are made up of a large number of membranes stacked between two electrodes. In these systems, the membranes alternate between cationic (passes cations) and anionic (passes anions) types. The chambers are filled alternately with a salt brine and a water solution to be desalted. Salts are pushed by the electric field from the water into the brine to produce a desalinated stream.
Salt-splitting cells (chlor-alkali cells) differ from electrodialysis cells primarily because their output is a base stream, an acid stream, or a generated gas instead of a purified water and a brine stream. Most salt splitting cells use only one or two membranes per cell, and large scale production is achieved by stacking many cells together.
Recent attempts to improve ECs have tried to increase turbulence. Turbulence has been increased by installing grids or "turbulence promoters." These turbulence promoters fill a gap between an electrode and a membrane so that fluid must flow around a series of obstructions. This causes a high degree of mixing which can break up boundary layers at membrane surfaces and remove bubbles which can occlude parts of the electrode. Usually the main advantage of turbulence promoters is an improvement in electrical efficiency. Potential drawbacks to turbulence promoters are that they foul easily and cause significant pressure drops. Fouling can be overcome by pretreating feed chemicals and pressure drops can be overcome by limiting the EC dimension. However, both solutions to fouling and pressure drops add considerable expense to the cost of an EC system.
One example of a cell designed to limit pressure drops while maintaining turbulence in the DEM.RTM. cell from electrocatalytic. In this cell, the electrodes are "dished out" over the active surface so that the flow path in its headers is relatively unrestricted. This avoids many of the flow distribution problems often found in narrow gap cells. Pressure drops can also be minimized in a lantern blade cell. The "lantern blade" term refers to an electrode made up of slats arranged in parallel and at an angle in a "Venetian blind" pattern. The membrane is supported by the edges of the pieces, and fluid flows in the gap in the slats.
ECs are used to produce materials which can also be obtained by other technologies. Therefore, economic considerations are of paramount concern in the design of ECs. Further, the greatest expense is electricity consumption. To minimize electrical resistance and electricity use, EC cells are usually designed with fluid compartments as thin as possible. For example, in ICI's two-chambered, lantern-blade electrocell (model FM21), the gap between electrodes is 2.5 mm. However, one drawback to narrow gap distances is that they limit fluid velocities through the EC (typically &lt;10 cm/sec). This exaggerates fouling problems.
Fouling problems are often not a major concern in commercial salt splitting applications because the feed streams can be pretreated to minimize fouling. However, pretreatment of feed streams adds considerably to expenses. Moreover, there are uncommercialized applications for ECs where fouling prevents commercial viability. These applications include processing streams which contain either suspended particulates or large gel-forming molecules in solution. Examples of such uncommercialized applications include desalting of liquid foods, removal of lignin from black liquor in wood pulp processing and desalting of pharmaceutical process streams.
The most rapid type of fouling is a surface phenomena. During EC operation, particles or large molecules in solution can adhere to the electrode and membrane surfaces due to electrical forces or due to differences in surface energies between the fluids and the surfaces. Eventually an impermeable film is formed over the surfaces. A method of suppressing this build up is to increase the amount of turbulence in the fluid. High sheer generated by turbulence can pull foulants away from the surface before they adhere to a surface and make a firm attachment. Therefore there is a need in the art for an EC which could maximize turbulence while maintaining small electrode spacing for processing fouling fluids.
High levels of turbulence can also increase the limiting current of an EC. "Limiting current" refers to a situation where ion flux through membranes is so fast that the concentration of ions near one side of a membrane becomes depleted. When this happens, the voltage gradient in the fluid near the membrane grows so steep that water begins to split into H.sup.+ and OH.sup.-. Such splitting consumes electrical energy without improving cell performance. Turbulence is, therefore, beneficial because it can delay the onset of water splitting by increasing mixing in the fluids and wiping away the ion-depleted layer. Other benefits of turbulence in ECs include the fact that bubbles that form on electrodes are stripped away quickly. Also minimizing ion depletion conditions near the membrane reduces voltage drops in the cell and lowers energy consumption.
Accordingly, there is a need in the art to find an alternate solution to membrane fouling problems that can allow the use of feedstock materials having a high suspended solids content, such as black liquor in the pulp and paper industry. The present invention was made to address these issues.